Current-to-voltage amplifiers are used widely in today's electronics. They are used with very small input currents such as 1.times.10.sup.-6 A to 1.times.10.sup.-11 A. Often, however, it is necessary to determine whether a current-to-voltage amplifier is functioning properly. This is particularly the case in nuclear instrumentation systems. It is very important to know if the active component of the amplifier, i.e., the operational amplifier ("op amp") is working properly.
A known current-to-voltage amplifier is shown in FIG. 1. In the normal mode of operation, switch S1 is closed and switch S2 is open. The gain of the amplifier is determined by resistor R8 with the relationship being V.sub.o =31 R8.multidot.I.sub.i. If R8 is variable or adjustable, such as by being a potentiometer or a set of resistors which alternately can be switched into the circuit, the gain of the amplifier becomes adjustable. R8 can have almost any value but typically is between 100.OMEGA. and 100M.OMEGA.. To monitor whether the amplifier is functioning properly, it can be operated in a test mode wherein switch S1 is open and switch S2 is closed. Depending upon the value of R8 and thus the gain of the amplifier, a test input current is applied by selecting from one of a series of voltage sources and corresponding resistors, V.sub.1 to V.sub.5 and R1 to R5, respectively. The values of V.sub.1 to V.sub.5 and R1 to R5 are varied so as to produce a range of test current values which when multiplied by R8 do not exceed the output capability of op amp 20.
The disadvantage with this test configuration is that the test input current chosen depends upon the gain of the amplifier. In normal operations, R8 is variable and is typically chosen from a set of resistors of many different values depending upon the range of the current being amplified. Depending upon what resistor value R8 currently has, a different test input current is required to test the operation of the amplifier without putting op amp 20 into saturation. As a result, several different voltage source values (V.sub.1-V.sub.5) and resistors (R1-R5) are needed to generate the different test currents. This of course, complicates the circuitry and increases the number of electronic components necessary. It also means that one must know the value of R8 before a test input current is applied so that the correct value can be chosen.
It would be desirable, therefore, to have a precision current-to-voltage amplifier which did not require a variety of test inputs, but used only one, and which, in the test mode of operation, had a fixed gain independent of the gain of the amplifier in the normal mode of operation.